Adult Critical Care Specialty (ACCS) Practice Exam

What is the formula used to calculate the patient's P/F ratio in the given scenario?

1. Plateau pressure divided by oxygen concentration

2. PaO2 divided by oxygen concentration

3. Mean airway pressure divided by arterial oxygen

4. Intracranial pressure divided by lung volume

The correct answer is: PaO2 divided by oxygen concentration

The formula used to calculate the patient's P/F ratio is indeed derived from the arterial blood gas analysis, specifically using the partial pressure of oxygen in arterial blood (PaO2) and the fraction of inspired oxygen (FiO2), which is often referred to as oxygen concentration in the context of this question. Therefore, the correct approach to determine the P/F ratio is to take the arterial oxygen partial pressure (PaO2) and divide it by the fraction of inspired oxygen (expressed as a decimal). This value provides important insights into the degree of hypoxemia and is commonly used in assessing the severity of respiratory failure or acute respiratory distress syndrome (ARDS). The P/F ratio is crucial for clinicians to evaluate how effectively oxygen is being delivered to the blood and to guide management strategies for patients with respiratory distress. A lower P/F ratio indicates more severe impairment in gas exchange, prompting further investigation and intervention. This understanding is essential in clinical practice, particularly in critical care settings. In the context of the other options, none accurately represent the formula for calculating the P/F ratio. Mean airway pressure and intracranial pressure are not relevant to this calculation, and plateau pressure is associated with ventilator management rather than oxygenation metrics.